The semantic minor mode changed the value of imenu-create-index-function on its startup, but does not restore the original value on its exiting. 

Here are the steps to reproduce the issue:

1. Start the emacs and open a python script, now the imenu-create-index-function will be the `python-imenu-create-index'

 $ emacs -nw -q ~/a.py   

2. Turn-on the semantic by "M-x semantic-mode", then the  imenu-create-index-function will be the `semantic-create-imenu-index'

3. Turn-off the semantic by "M-x semantic-mode",  imenu-create-index-function is still the semantic one.

This patch will store the original imenu-create-index-function before changing it, and restore the original value when turn-off the semantic-mode.